Bhashya
‘द्वया ह’ इत्याद्यस्य कः संबन्धः? कर्मणां ज्ञानसहितानां परा गतिरुक्ता मृत्युवात्मभावः, अश्वमेधगत्युक्त्या ।अथेदानीं मृत्युवात्मभावसाधनभूतयोः कर्मज्ञानयोर्यत उद्भवः, तत्प्रकाशनार्थमुद्गीथब्राह्मणमारभ्यते ॥
In plain words
What is the connection of the passage beginning with 'There are two indeed'? Through the teaching on the goal of the horse-sacrifice, the highest attainment for actions united with knowledge was described—namely, reaching the state of death. Now, to reveal the source and origin of these very means of action and knowledge that lead to that state, the Udgītha Brāhmaṇa is introduced.
Close to the Sanskrit
What is the connection of that beginning with 'There are two indeed'? The supreme goal of actions combined with knowledge—namely, attaining the nature of death—has been stated through the mention of the destination of the horse-sacrifice. Now, for the purpose of illuminating the origin of action and knowledge, which are the means for attaining the state of death's self, the Udgītha Brāhmaṇa is commenced.